1. Field of the Invention
The present invention pertains to a porous media type air filter unit having a multiple jet reverse flow cleaning system for the filter element wherein a substantial volume of jet cleaning air and entrained ambient air are used to flush the filter element to remove accumulated dust particles.
2. Background
In the art of impingement or porous media barrier type air filters, systems have been developed for removing accumulated dust or "cake" from the surface of the filter element by jetting a pulse of pressure air from the clean air or downstream side of the filter element to shock the dust from the element surface. The configuration of conventional filter elements, being a substantially cylindrical annular pleated paper or cloth bag type structure, has resulted in the evolution of a reverse flow cleaning system wherein a jet nozzle is disposed at one end of the filter element somewhat downstream of the inner clean air chamber defined by the element and aimed in the opposite direction to that of the clean air flow out of the element itself. The jet nozzle is typically connected to a source of pressure air through a timer controlled valve wherein, periodically, a short burst or pulse of pressure air is discharged through the nozzle to create a pressure wave which flows through the filter element in reverse direction to remove the dust which has accumulated on the surface of the element itself.
The conventional design of reverse flow cleaning systems relies on the concept of delivering a relatively high pressure pulse of pressure air in the range of 80-100 psig and of very short duration on the order of a few milliseconds, or less, to create a pressure wave. This brief pulse of air is of such short duration that the air jet cannot adequately entrain ambient air surrounding the jet in order to deliver a substantial quantity of reverse flow air to thoroughly flush the filter element clean instead of removing only some of the dust free by the short pulse or pressure wave. Thus, for the amount of energy expended in the high pressure pulse type reverse flow cleaning systems a relatively inefficient process is carried out and the pressure wave unduly stresses the filter media. In order to overcome the problems with prior art systems certain types of barrier filter elements such as the porous media pleated paper type have been grossly derated as to their filtering capacity to prevent early filter element deterioration and failure due to the reverse pulse jet action. One prior art type of filter cleaning system using porous media elements requires derating of the element for most applications to a maxima capacity of 2-3 cubic feet per minute of filter air flow per square foot of filter surface area.
Another problem associated with prior art type reverse flow filter cleaning systems is the amount of space occupied by the reverse flow jet nozzle and associated structure, particularly as the size of the filter element itself increases. Since the cabinet structure around the filter element and the element itself are desirably kept at a minimum size for a given filtering capacity, prior art pulse type air jet cleaning systems have occupied a disproportionately large amount of space in the filter cabinet structure which is not used to any other advantage. The capabilities of prior art reverse jet filter cleaning apparatus has also limited the filter flow area for a given filtering capacity and the flow area of the clean air discharge conduit or flow tube associated with the filter element. Moreover, known types of reverse jet filter cleaning systems have been confined to use with filter elements which are substantially cylindrical tubular structures since the pulse delivered by a single jet nozzle will not conform to the shape of an irregular or noncircular filter cross-sectional shape and produce effective cleaning of the entire filter surface.
The aforementioned problems in the art of reverse jet cleaning systems for air filters, together with the recognition of the need to provide a filter structure which has a maximum filtered air flow area, provides for a minimum filtered air pressure drop for a given filtering requirement, and a more thorough reverse flow cleaning system which will minimize stress on the filter media, has led to the pursuit of a system which will produce relatively large volumes of low pressure air flow through the filtering medium in a reverse direction to completely flush the filter element itself. A system which meets these objectives and overcomes several problems in the art is provided by the present invention.